1. Field of the Invention
This invention concerns strippers used with punch holders to enable withdrawal of a punch from a workpiece after the punch has penetrated the workpiece.
2. Description of the Relevant Art
Punch holders are often equipped with a stripper, which comprises a workpiece engaging element having an opening through which the punch passes as it is advanced to penetrate the workpiece. After penetration, the stripper element is held against the workpiece surface by stripper springs as the punch is withdrawn. This insures that the workpiece is not pulled up with the punch if the workpiece grips the punch about the perimeter of the punched opening. The punch is thereby stripped from the workpiece.
Stripper openings are closely matched to the punch shape so as to maintain the flat shape of the workpiece about the perimeter of the punched opening. Thus, strippers must be changed with changes in punches in order to match the stripper punch opening to the particular punch shape.
The mounting of the stripper element should accordingly allow easy removal and installation. Designs which use small loose parts such as screws, snap rings, etc., which are slow to be installed are disadvantageous for this reason.
U.S. Pat. No. 4,646,599 issued Mar. 3, 1987 for a "Hydraulic Punch Press with Workpiece Stripper"; U.S. Pat. No. 4,989,484 issued on Feb. 5, 1991, for a "Punch and Stripper Assembly"; and U.S. Pat. No. 3,530,750 issued on Sep. 29, 1970, for a "Punching Tool Assembly", each show stripper arrangements requiring such extra parts which must be installed.
Soft materials such as urethane plastic must sometimes be employed for the stripper, which is usually of steel, to avoid marring workpieces of such metals as aluminum or brass. For this reason the stripper arrangement employed should allow ready replacement of strippers of steel with those of plastic materials without necessitating the use of a different punch holder.
U.S. Pat. No. 5,176,057 issued on Jan. 5, 1993, for a "Punch Holder with Stripper Arrangement", assigned to the same assignee as the present application, describes a stripper arrangement which allows stripping elements of either steel or plastic to be readily replaced and which does not employ loose parts.
However, the mounting of the stripper element is a sleeve or cap held on the holder with friction generated by contact of an O-ring carried by the holder. This arrangement is not sufficiently secure to resist pulling forces that are sometimes developed, as when an oil film is present causing adhesion to the workpiece. This can cause the stripper sleeve to be pulled off.
It would also be advantageous if steel stripper elements could be made even simpler and lower in cost.